1. Technical Field
One or more embodiments of the present invention relate to a surface light source device and, more particularly to the structure of a surface light source device for letting light efficiently enter a light guide plate thinner than the thickness of a light source.
2. Background Art
FIG. 1 is a schematic diagram of a conventional liquid-crystal display device using an edge-light type surface light source device. This liquid-crystal display device 11 is configured of a surface light source device 12 and a liquid crystal panel 15.
In the surface light source device 12, a point light source 18 using an LED is disposed so as to face an end face (a light incidence surface) of a light guide plate 17 molded of transparent resin, a diffusion plate 13 and two prism sheets 14 are stacked on an upper surface (a light exit surface) of the light guide plate 17, and a reflecting plate 16 faces a lower surface of the light guide plate 17. Note that the point light source 18 is implemented on a substrate 20. The liquid crystal panel 15 is disposed on one of the prism sheets 14 via a rim sheet 19 (a black frame).
As such, light emitting from the point light source 18 enters the light guide plate 17 from the end face thereof, spreads to be propagated through the light guide plate 17, and exits from an approximately entire upper surface of the light guide plate 17. The light exiting from the upper surface of the light guide plate 17 is transmitted through a diffusion plate 13 and a prism sheet 14 to illuminate the liquid crystal panel 15 from its rear surface. Also, light leaking from a lower surface of the light guide plate 17 is reflected from the reflecting plate 16 to return to the inside of the light guide plate 17 again for reuse.
In this surface light source device 12, in addition to uniform luminance, high luminance and low cost, furthermore a large light-emitting area (an area other than the light exit surface is small) and thin thickness are desired. In particular, when incorporated in a portable device, the surface light source device 12 has been increasingly desired to be made thinner, as the portable device is made thinner.
The size of each component of a surface light source device generally used is as follows.                Sum of the thicknesses of the substrate and the point light source: 600 μm        Height of the light exit window of the point light source: 300 μm        Thickness of the prism sheet: 62 μm (per sheet)        Thickness of the diffusion plate: 55 μm        Thickness of the light guide plate: 250 μm to 650 μm        Thickness of the reflecting plate: 60 μm        Thickness of the rim sheet: 55 μm        
Thus, the thickness of the surface light source device is on the order of 600 μm on a point light source side, and approximately 489 μm to 889 μm on a light guide plate side even when the thickness of a rim sheet is excluded. Therefore, the thickness on a light guide plate side occupying most of the area of the surface light source device is desired to be made thinner.
Most of the thickness of the surface light source device is occupied by the light guide plate. However, if the thickness of the light guide plate is made thinner than the height of a light exit window (a light-emitting window) of the point light source, light not incident to the light guide plate of light exiting from the point light source is increased, and light use efficiency of the surface light source device is decreased. For this reason, the thickness of the light guide plate is restricted by the height of the light exit window of the point light source, and it is difficult to make the thickness of the light guide plate thinner than the height of the light exit window of the point light source. Similarly, when the light source is a cold-cathode-fluorescent tube, it is difficult to make the thickness of the light guide plate thinner than a diameter of a cold-cathode-fluorescent tube.
(Regarding Patent Document 1)
FIG. 2 is a side view of a liquid-crystal display device 21 disclosed in Japanese Unexamined Patent Application Publication No. 1993-53111 (Patent Document 1). In a surface light source device 22 for use in this liquid-crystal display device 21, to efficiently let light from a fluorescent tube 23 enter a light guide plate having a thickness thinner than that of the fluorescent tube 23, a tapered part 25 is provided at a thin portion of the light guide plate, that is, at an end of a light guide plate body 24. An end face of the tapered part 25 has a height approximately equal to the diameter of the fluorescent tube 23, and the fluorescent tube 23 faces that end face. And, the light entering from the end face of the tapered part 25 is totally reflected from front and rear surfaces of the tapered part 25 to be guided to the light guide plate body 24, and is output from an upper surface of the light guide plate body 24 toward a liquid crystal panel 26.
The surface light source device 22 disclosed in Patent Document 1 discloses guiding the light of the fluorescent tube 23 to the light guide plate without leakage. For this purpose, the height of the end face of the tapered part 25 is made approximately equal to the diameter of the fluorescent tube 23, thereby introducing the light of the fluorescent tube 23 into the tapered part 25 without leakage. However, in this surface light source device 22, leakage of light in the tapered part 25 cannot be prevented. For this reason, light leaking from the tapered part 25 is viewed brightly from an observer's side, and the edge of a display part (a screen) of the liquid-crystal display device 21 emits light with high luminance, thereby degrading the quality of the display part.
The reason why leakage of light from the tapered part 25 cannot be prevented in the structure of the surface light source device 22 as described above is described by using FIG. 3. Now, consider light that tends to leak most at the tapered part 25. If this light that tends to leak most can be prevented, this means no leakage of light at the tapered part 25. The light that tends to leak most is a light beam L having the largest incident angle α among light beams output from the fluorescent tube 23 and entering the tapered part 25. Therefore, consider a structure in which the light beam L having the largest incident angle α measured from a direction perpendicular to the end face of the tapered part 25 does not leak at the tapered part 25 and the thickness of the light guide plate body 24 can be made as thin as possible. To find the structure as described above, as depicted in FIG. 3, a condition can be thought for the light beam L having the maximum incident angle α being totally reflected at an upper end (a point A) of the inclined surface of the tapered part 25, being totally reflected again at a point B on a lower surface of the light guide plate, and then being reflected from an upper surface (at a point C) of the light guide plate body 24 adjacent to the tapered part 25. Note that, in FIG. 3, a flat short part is depicted at the end face portion of the tapered part 25, but this is merely for convenience of representation, and its length can be thought to be short to infinity.
First, the maximum incident angle α of the light beam incident to the light guide plate is determined bysin α=1/n  (Equation 1)
(where n is a refractive index of the light guide plate).
An incident angle at which this light beam L having the largest incident angle α enters the point A having an inclination angle θ in the tapered part 25 is 90°−θ−α, and therefore the condition of the light beam being totally reflected from the inclined surface isθ≦90°−2α  (Equation 2).
Also, an incident angle at which the light totally reflected at the point A enters the lower surface of the tapered part 25 is 90°−2θ−α, and therefore the condition of the light beam being totally reflected at the point B on the lower surface isθ≦45°−α  (Equation 3).If this Equation 3 is satisfied, the light totally reflected at the point B is also totally reflected at a point C of the light guide plate body 24.
Therefore, from Equation 2 and Equation 3, for the light beam L to be totally reflected at the point A, the point B and the point C, it can be found thatθ≦45°−α  (Equation 4)is satisfied. However, if the inclination angle θ of the tapered part 25 is small, the light totally reflected at the upper end of the inclined surface of the tapered part 25 and then totally reflected from the lower surface of the light guide plate again enters the inclined surface of the tapered part 25 to possibly leak from the tapered part 25. Moreover, if the inclination angle θ is small, the length of the tapered part 25 is long. Therefore, the inclination angle θ may be as long as possible within a range of satisfying Equation 4. Therefore, the inclination angle θ is assumed to have a value as large as possible within a limit of satisfying Equation 4. That is,θ=45°−α  (Equation 5)
And, when the height of the end face of the tapered part 25 is T, the length of the tapered part 25 is X, and a height difference of the inclined surface of the tapered part 25 is Y, the length X and the height difference Y of the tapered part 25 are, from FIG. 3,X=T cot(α+2θ)+(T−Y)cot(α+2θ)=(2T−Y)cot(α+2θ),Y=X tan θ.These are solved for X and Y, and when Equation 5 is used, the following Equation 6 and Equation 7 hold.
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            X        =                              2            ⁢                          a              ⁡                              (                                  1                  +                  a                                )                                      ×            T                                1            +                          2              ⁢              a                        -                          a              2                                                          (                  Equation          ⁢                                          ⁢          6                )                                          Y          =                                    2              ⁢                              a                ⁡                                  (                                      1                    -                    a                                    )                                            ×              T                                      1              +                              2                ⁢                                                                  ⁢                a                            -                              a                2                                                    ⁢                                  ⁢        where        ⁢                                  ⁢                  a          =                                    tan              ⁢                                                          ⁢              α                        =                          tan              ⁢                                                          ⁢                              (                                                      45                    ⁢                    °                                    -                  θ                                )                                                                        (                  Equation          ⁢                                          ⁢          7                )            
Also, a thickness t of the light guide plate body 24 is represented by the next Equation 8.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                            t        =                              T            -            Y                    =                                                    (                                  1                  +                                      a                    2                                                  )                            ×              T                                      1              +                              2                ⁢                a                            -                              a                2                                                                        (                  Equation          ⁢                                          ⁢          8                )            
As a light guide plate material, acrylic resin or polycarbonate resin (PC resin), both of which are typical materials for light guide plates, is assumed. When a refractive index of the light guide plate is taken for calculation as
n=1.49 (in the case of acrylic resin) or
n=1.59 (in the case of polycarbonate resin),
the maximum incident angle α is, from Equation 1,
α=42.16° (in the case of acrylic resin) or
α=38.97° (in the case of polycarbonate resin).
From Equation 3, the inclination angle θ of the tapered part 25 is
θ=2.84° (in the case of acrylic resin) or
θ=6.03° (in the case of polycarbonate resin).
Also, in Patent Document 1, the height of the end face of the tapered part 25 is described as T=4.10 mm. Thus, by using this value of the height T and the value of a above, from Equation 6 to Equation 8, the length X and the height difference Y of the tapered part 25 and a thickness t of the light guide plate body 24 areas follows. When the light guide plate material is acrylic resin, T=4.10 mm and α=42.16° (α=tan α=0.91), and therefore
X=7.10 mm,
Y=0.35 mm, and
t=3.75 mm.
Similarly, when the light guide plate material is polycarbonate resin, T=4.10 mm and α=38.97° (α=tan α=0.81), and therefore
X=6.11 mm,
Y=0.65 mm, and
t=3.45 mm.
FIG. 4 summarizes the calculation results as described above.
According to FIG. 4, the thickness t of the light guide plate body 24 is 3.75 mm (in the case of acrylic resin) or 3.45 mm (in the case of polycarbonate resin). By contrast, in the liquid-crystal display device 21 disclosed in Patent Document 1, it is described that, while the height of the end face of the tapered part 25 is T=4.10 mm, the thickness of the light guide plate body 24 is t=2.2 mm. This value t=2.2 mm is considerably thinner than the value of the thickness t found by the above calculation (in FIG. 4), and therefore light always leaks from the tapered part 25.
Therefore, in the surface light source device 22 disclosed in Patent Document 1, it is impossible to prevent light from leaking form the tapered part 25. Or, in the surface light source device 22 disclosed in Patent Document 1, at least leakage of light from the tapered part 25 is not considered at all.
(Regarding Patent Document 2)
FIG. 5 is a perspective view of a surface light source device disclosed in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2004-69751 (Patent Document 2). In this surface light source device 31, a cone-shaped light guiding part 33 is provided at an end of a light guide sheet 32, and a point light source 35 is disposed so as to face an end face (a light receiving part 34) of the light guiding part 33. Also in this surface light source device 31, the point light source 35 and the light receiving part 34 of the light guiding part 33 are approximately equal in height to each other, and light from the point light source 35 enters into the light guiding part 33 to be guided to the light guide sheet 32.
The surface light source device 31 disclosed in Patent Document 2 discloses guiding the light of the point light source 35 to the light guide sheet 32 without leakage. For this purpose, the height of the light receiving part 34 is approximately equal to the height of the point light source 35 and, with the tapered part of the light guiding part 33, light of the point light source 35 is guided to the light guide sheet 32 without leakage. However, even in this surface light source device 31, leakage of light in the cone-shaped light guiding part 33 cannot be prevented. For this reason, light leaking from the light guiding part 33 is viewed brightly from an observer's side, and the edge of a display part (a screen) of the liquid-crystal display device emits light with high luminance, thereby degrading the quality of the display part.
FIG. 6 is a diagram depicting a section of the light guiding part 33 and the light guide sheet 32 cut along a vertical plane passing an axial center of the light guiding part 33. By using FIG. 6, the fact that leakage of light from the light guiding part 33 cannot be prevented even in the surface light source device 31 of Patent Document 2 will be described. In the surface light source device 31 of Patent Document 2, because acrylic resin is used as the light guide plate material, a maximum incident angle α of a light beam L entering the light guiding part 33 is α=42.16° from FIG. 4. At this time, an inclination angle θ of the front surface of the light guiding part 33 is θ=2.84°. However, in the surface light source device 31 of Patent Document 2, because a height T of the light receiving part 34 is 0.3 mm and a thickness t of the light guide sheet 32 is 0.1 mm, when the inclination angle of the light guiding part 33 is θ=2.84°, the length of the light guiding part 33 is X=2.2 mm. Thus, as depicted in FIG. 6, light totally reflected from the upper end of one inclined surface of the light guiding part 33 enters the other inclined surface at an incident angle of90°−(α+30)=39.32°.Because this incident angle of 39.32° is an angle smaller than a critical angle (42.16° of total reflection, the light beam L entering the other inclined surface leaks to outside as depicted in FIG. 6.
Therefore, in the surface light source device 31 disclosed in Patent Document 2, although light can be confined to some extent, but approximately several tens of percent of light leaks from the light guiding part 33, and leaked light lights up brightly on the display surface to degrade the quality of the liquid-crystal display device. Also, in Patent Document 2, prevention of this leakage of light is not considered at all. Moreover, in the surface light source device 31 of Patent Document 2, the end face of the cone-shaped light guiding part 33 and the point light source 35 are disposed so as to face each other. Thus, the position of the light guiding part 33 in the light guide sheet 32 has to be changed depending on the position or size of the point light source 35, and therefore the device lacks versatility.
(Regarding Patent Document 3)
FIG. 7 is a partially-cut-out perspective view of a surface light source device disclosed in Japanese Patent No. 3828402 (Patent Document 3). In this surface light source device 41, a diffusion plate 43 and two prism sheets 44 and 45 are stacked on a light guide plate 42, and a plurality of point light sources 46 are disposed so as to face a light incidence end face of the light guide plate 42. On upper and lower surfaces of a region along the light incidence end face of the light guide plate 42, a plurality of optical directivity diffusion elements 47 extending in a direction perpendicular to the light incidence end face are provided, letting light entering from the point light sources 46 be transmitted along a flat surface of the light guide plate 42 as being diffused along the flat surface. Furthermore, on the upper and lower surfaces of the light guide plate 42, a plurality of dots 48 are provided to let light guided inside the light guide plate 42 exit to outside.
This surface light source device 41 can mitigate a glow of light with high luminance leaking from near the point light sources. In this surface light source device 41, however, the light guide plate 42 has a uniform thickness as a whole. Thus, if incident efficiency of light of each light source is intended not to be decreased, the thickness of the light guide plate 42 cannot be made thinner than the height of the light-emitting surface of the light source.
(Regarding Patent Document 4)
FIG. 8 is a perspective view of a surface light source device disclosed in Japanese Unexamined Patent Application Publication No. 2003-272428 (Patent Document 4). In this surface light source device 51, a plurality of light sources 53 are disposed so as to face a light incidence end face of a light guide plate 52, and a step surface 54 perpendicular to an upper surface of the light guide plate 52 is formed in a zigzag shape so as to face a upper half surface of the light incidence end face, thereby increasing the thickness of the light guide plate 52 on a light incidence end face side and decreasing the thickness of a main part of the light guide plate 52 away from the light incidence end face.
In the above-structured surface light source device 51, light entering from the upper half surface of the light incidence end face to the inside of the light guide plate 52 is regressively reflected from the step surface 54 to return to the incident end face and, by being reflected from the incident end face, the light is guided to a region of the light guide plate having a thin thickness. Thus, the surface light source device can be made thinner, and light use efficiency can be increased.
However, in this surface light source device 51, as depicted in FIG. 9A, depending on the incident angle to the step surface 54, light L is not totally reflected from the step surface 54 to leak to outside and become a loss. Even when a reflecting member 55 is provided to face the step surface 54, part of the leaked light L is absorbed in the reflecting member 55 to become a loss. Moreover, as depicted in FIG. 9B, the light L regressively reflected from part of the step surface 54 partially returns to the inside of the light source 53, and is absorbed by a packaged of the light source 53 to become a loss. For this reason, in the surface light source device 51, light use efficiency is low, although the light guide plate can be made thinner.
Still further, in this surface light source device 51, the zigzag shape of the step surface 54 is required to be formed so as to match the position of each of the light sources 53. When the number or position of the light sources 53 is changed, the light guide plate 52 is required to be redesigned or recreated. Still further, in this surface light source device 51, the shape of the light guide plate 52 is complex and difficult to manufacture.